The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a laser beam printer, and more particularly to an image forming apparatus of such a type that a toner image formed on an image bearing member such as a photosensitive drum is transferred onto a recording sheet via an intermediate transfer member.
Generally, in image forming apparatuses such as electrophotographic copying machines and laser beam printers, a toner image is formed on the surface of a photosensitive drum according to an image data. Then, the toner image is transferred onto a recording sheet, and the transferred toner image is heat-fixed to the recording sheet. Accordingly, a recording image is obtained. In recent year, there are also known full-color copying machines and full-color laser beam printers in which toner images formed on photosensitive drums are primarily transferred onto an intermediate transfer member, the toner images in four colors of yellow, cyan, magenta, and black are overlapped with each other on the intermediate transfer member, and then a resultant composite toner image is collectively secondarily transferred onto a recording sheet, to thereby form a full-color recorded image.
It is difficult to maintain a transfer efficiency for a toner image at 100%, so that an untransferred toner is likely to remain on the surface of the photosensitive drum after the toner image is transferred onto the recording sheet or the intermediate transfer member. Therefore, up to now, it is customary to provide a cleaning device on the downstream side of a transfer portion of the toner image and remove a residual toner prior to formation of the next toner image. Mainstream examples of such a cleaning device include a device in which a toner is scraped off from the surface of the photosensitive drum by a scraper (JP 06-148910 A) and a device in which a brush roll that rotates at high speed is made to slidingly contact the surface of the photosensitive drum (JP 04-29283 A). However, these cleaning devices have a defect in that the life of the photosensitive drum is likely to be shortened due to mechanical wear.
From this viewpoint, in recent years, there has been introduced an image forming apparatus in which, by increasing the transfer efficiency for a toner image to reduce the cleaning load on the photosensitive drum, the mechanical wear of the photosensitive drum is reduced (JP 2001-075448 A). More specifically, a spherical toner excellent in transfer efficiency is used, a rigid intermediate transfer drum is used as the intermediate transfer member onto which a toner image is primarily transferred, and an elastic layer whose resistivity is adjusted is provided on the surface of the intermediate transfer drum. Accordingly, the transfer efficiency for a toner image is increased and it becomes possible to continuously form images without providing the cleaning device for the photosensitive member.
On the other hand, the transfer efficiency for the toner image from the intermediate transfer drum onto the recording sheet is affected by environmental factors such as temperature and humidity, the type of the recording sheet, the moisture content of the recording sheet, and the like. Therefore, it is more difficult to increase the transfer efficiency of the intermediate transfer drum than that of the photosensitive drum. Also, in the case of a so-called tandem color image forming apparatus including the photosensitive drums for the colors of yellow, magenta, cyan, and black, the toner images in the respective colors are overlapped on the intermediate transfer drum. Therefore, the total amount of the toners thereon to be transferred is larger than the amount of the toners on the photosensitive drum. Accordingly, a larger amount of untransferred toners are likely to remain on the intermediate transfer drum than on the photosensitive drum, and the untransferred residual toners appear as a ghost image in the recorded image, so that there is high demand for providing a cleaning device for the intermediate transfer drum.
However, the elastic layer is formed on the surface of the intermediate transfer drum so as to increase the transfer efficiency between the intermediate transfer drum and the photosensitive drum. Thus, there is a problem in that, if the scraper is brought into pressure contact with the surface of the intermediate transfer drum, the elastic layer is likely to be damaged. Particularly, it is difficult to stably remove a spherical toner excellent in transfer efficiency by using a scraper formed of an elastic material such as a urethane rubber. Thus, it is unavoidably necessary to use a metallic scraper formed of an SUS or the like. If such a metallic scraper is used, the surface of the elastic member is damaged, and there is an extremely high probability of deteriorating the transfer efficiency.
Also, if the intermediate transfer drum is cleaned by using a brush roll, there is a problem in that irregularities are likely to occur in the elastic layer on the surface of the intermediate transfer drum due to its use over time, which leads to reduction in the transfer efficiency for a toner image.
The present invention has been made in view of the above problems, and an object of the present invention is therefore to provide an image forming apparatus capable of reliably removing an untransferred residual toner from the surface of an intermediate transfer drum which is covered with an elastic layer, thereby enabling to form a recorded image having a high image quality with no ghost image, and capable of continuing to form recorded images having the high image quality over a long period of time without causing damage to the intermediate transfer drum.
In order to achieve the above-mentioned object, an image forming apparatus of the present invention is premised to include: an image bearing member on which a toner image corresponding to image information is formed; a final transfer roll for transferring the toner image onto a recording sheet; at least one intermediate transfer member for mediating a transfer of the toner image from the image bearing member onto the recording sheet, which is disposed between the image bearing member and the final transfer roll; and a toner discharging member for capturing a toner adhered on the intermediate transfer member and discharging the toner as waste into a recovery box, which is disposed to oppose the intermediate transfer member, and is characterized in that: the intermediate transfer member is structured as an intermediate transfer drum in which a periphery of a metal base formed into a cylindrical shape is covered with an elastic layer; and the toner discharging member has: a recovery roll that rotates while being in contact with the intermediate transfer drum; and a scraper for scraping off a toner adhered to a surface of the recovery roll.
In the above technical measure, there occurs no problem even if a peripheral speed of the recovery roll is the same as a peripheral speed of the intermediate transfer drum. From the viewpoint of removing the toner adhered on the intermediate transfer drum more efficiently, it is preferred that the peripheral speed of the recovery roll be different from that of the intermediate transfer drum. However, if the peripheral speed difference is excessively large, there occurs a so-called filming phenomenon in which the toners are rubbed over the surface of the intermediate transfer drum and thinly fixed on the surface. Therefore, it is preferred to set the above peripheral speed difference within 2% of the peripheral speed of the photosensitive drum.
Further, the recovery roll may be structured such that a metallic roll is directly brought into pressure contact with the intermediate transfer drum, or a surface of the metallic roll may be covered with the elastic layer. However, if a spherical toner is used as the toner, a metal plate is suitable for the scraper for scraping off the spherical toner adhered to the recovery roll. Thus, when taking into account the damage to the recovery roll due to the metallic scraper, it is preferred that the recovery roll itself be merely the metallic roll.
In addition, there occurs no problem as long as the recovery roll is in contact with the intermediate transfer drum at a certain level of pressure and the toner on the intermediate transfer drum is mechanically transited thereto. However, from the viewpoint of removing the toner on the intermediate transfer drum more efficiently, it is preferred to structure the apparatus such that a potential gradient is formed between the intermediate transfer drum and the recovery roll and the toner is electrically transited from the intermediate transfer drum to the recovery roll based on the potential gradient.
As a measure for forming the potential gradient between the intermediate transfer drum and the recovery roll, from the viewpoint that the transfer bias is applied to the intermediate transfer drum, it is preferred that the recovery roll be grounded via a resistor and a potential difference based on the resistance is formed between the intermediate transfer drum and the recovery roll. Also, it is conceivable to apply a cleaning bias directly to the recovery roll.
On the other hand, the untransferred and unnecessary toners remaining on the intermediate transfer drum include, in addition to the normal-polarity toner, a toner whose polarity is inverted due to Paschen discharge or charge injection in a transfer nip when passing through a transfer electric field, that is, a reversed-polarity toner. Therefore, if the cleaning bias is applied to the recovery roll, the toner that can be electrically captured due to the cleaning bias is a toner with only one of the polarities. Thus, in order to remove unnecessary toners with both the normal polarity and the reversed polarity from the surface of the intermediate transfer drum, there is provided a cleaning mode which is executed while an image forming operation is not executed, that is, before or after the image forming operation is executed. It is preferred to structure the apparatus such that the potential gradient formed between the intermediate transfer drum and the recovery roll is reversed between a case of the image forming operation and a case of executing the cleaning mode. Accordingly, by adopting a structure in which the potential gradient is reversed between the case of the image forming operation and the case of the cleaning mode, during the image forming operation, one of the normal-polarity toner and the reversed-polarity toner is transited from the intermediate transfer drum to the recovery roll. On the other hand, during the cleaning mode, the toner with the other polarity can be transited from the intermediate transfer drum to the recovery roll. At the end of the cleaning mode, both the normal-polarity toner and the reversed-polarity toner which are adhered to the intermediate transfer drum can be removed from the surface of the intermediate transfer drum.
Further, the reversed-polarity toner remaining on the intermediate transfer drum is transited backward along a potential gradient from the intermediate transfer drum to the photosensitive drum. Thus, if the reversed-polarity toner is not removed from the surface of the intermediate transfer drum during the image forming operation, the photosensitive member becomes contaminated with the reversed-polarity toner, and finally the charger for charging the photosensitive member becomes contaminated, which will lead to not only uneven charging of the photosensitive member but also uneven density of the recorded image. Accordingly, from the viewpoint of removing both the normal-polarity toner and the reversed-polarity toner from the intermediate transfer drum during the image forming operation and forming a recorded image with a high quality having no uneven density, the following structure is preferable. That is, a toner temporarily-holding member for electrically capturing a toner adhered to a surface of the intermediate transfer drum, which is in contact with the surface of the intermediate transfer drum, is provided on the upstream side of the toner discharging member with respect to a rotational direction of the intermediate transfer drum. During the image forming operation, the toner discharging member and the toner temporarily-holding member each remove a toner having a polarity reverse to each other from the surface of the intermediate transfer drum. When the cleaning mode is executed, the toner captured by the toner temporarily-holding member is transited to the intermediate transfer drum to be removed by the toner discharging member. In the above structure, all the toners of both polarities that remain on the intermediate transfer drum after transferring the toner image can be removed during the image forming operation, thereby enabling to prevent not only the contamination of the charger due to the reversed-polarity toner but also the uneven density of the recorded image. Also, it is unnecessary to provide a toner discharging member for each of the normal-polarity toner and the reversed-polarity toner, thereby enabling to downsize the image forming apparatus.